A coupling device for joining plastic pipe is disclosed in U.S. Pat. No. 3,877,735, the disclosure of which is hereby incorporated by reference. U.S. Pat. No. 3,908,261 also discloses the coupling device in the '735 patent. The coupling device disclosed in the '735 and '261 patents includes a pair of tubular insert members which are inserted into opposed ends of plastic pipe sections, a pair of semi-circular retaining clips which fit over abutting flanges of the insert members, a tubular coupling member which fits over the outer surfaces of the pipe sections and a pair of rigid rings which slide toward each other and over the outer surface of the coupling member to plastically deform the inside of the plastic pipe sections into interlocking relationship with serrations on the outer surfaces of the insert members. The inserts, clips and rigid rings are formed of metal and the coupling member is made of a deformable plastic.
The coupling member described above suffers certain drawbacks. For instance, when the rigid rings are pressed toward each other to complete the coupling, the direction of plastic flow of the plastic pipe sections is the same as the rigid ring motion. That is, the plastic pipe sections tend to flow toward each other. However, due to lack of free space between the ends of the pipe sections and the retaining clips, a large force is required to slide the rigid rings toward each other. That is, since there is no free space to accommodate flow of the plastic pipe sections, a large insertion force is necessary to compress the deformable coupling member and deform the plastic pipe between the rigid rings and the insert members. The worst side effect of this design is the phenomenon called "bunching" and excessive ring stress could lead to corrosion failure of the coupling. Accordingly, there is a need in the art for a coupling member which avoids the disadvantages of the prior art coupling.